I'm having a similar problem, not exactly the same, but i'll post my symptoms just in case it helps someone else.

My setup is Ubuntu 8.10 32-bit, but running the server kernel. Using VirtualBox 2.1.0, and an XP SP2 guest. Every now and then, the XP guest will just freeze, and stop resaponding to inputs, needs to be reset. The little network icon on the VirtualBox window frame shows some occassional activity even though the guest is frozen.

Any, so far, it only seems to happen when 1) I have Host Interface Networking configured, and 2) I am trying to install a new application. When both of these things are true, it will freeze 75% of the time.

I'm still investigating this to see if I can see further patterns, anyone else seen anything similar?